It is the processes of formation of erythrocytes, leukocytes and platelets in the body. Formation of erythrocytes and leukocytes respectively are known as erythropoiesis and leukopoiesis.
During embryonic state the blood islands of pander of the yolk sac functions as a site of hematopoiesis. The mesenchymal cells of the liver, spleen, bone marrow and lymph glands are the hemopoietic organs in early fetal life.
The bone marrow is concerned with the production of erythrocytes, granulocytes and platelets during postnatal life, whereas the lymphocytes production occurs in the lymphoid tissues of lymph glands, Payer's patches of intestine, spleen and thymus.
The lymphoid tissues of the bone marrow and also the spleen are the sites of production for monocytes. In ruminants, hemolymph nodes (hemal) functions as spleen. It takes part in the erythropoiesis during the foetal period, while granulopoiesis, is more prevalent in postnatal life.
The mesenchymal cells of the yolk sac produce primitive stem cells, which give rise to the pleuripotent stem cells (colony forming units - CFU). These stem cells give rise to five different blast cells, viz.
Proerythroblast to form RBC
Myeloblast to form neutrophils, eosinophils and basophils
Monoblast to form monocyte
Lymphoblast to form lymphocyte
Megakaryoblast to form platelets. Depending on the microenvironment, i.e., the location of the stem cells and the growth factors, the stem cells differentiate into progenitor cells of different blood cells (Committed Stem Cells-CSC). A CSC that produces erythrocytes is called colony-forming unit-erythrocyte (CFU-E). Similarly, CFU that produce granulocytes and monocytes are designated as CFU-GM.
The stem cells continue to divide throughout the life of the animal and a part of the cells remains as pleuripotent stem cells and retained in the bone marrow to maintain supply of stem cells.
The pleuripotent stem cells differentiate to form the CSC. Several hemopoietic growth factors and differentiation factors stimulate the growth and differentiation of these stem cells into a particular progenitor cells.
Cytokinins are the growth factors that regulate the formation of blood cells. Two cytokinins that stimulate red cell and WBC formation are the colony stimulating factors and interleukins. Erythropoietin increases erythrocyte precursor formation.
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Erythropoeisis:-
From stem cell, the formation of reticulocyte takes about 72 hours and conversion of reticulocyte to erythrocyte requires 48 hours. Thus RBC formation requires 5 days time.
Under appropriate stimulation, CFU-E progenitor cells produce proerythroblast. Hb synthesis begins in polychromatophil erythroblast and maximum synthesis occurs in orthochromatic erythroblast.
The metarubricyte ejects the nucleus to become the reticulocyte that contains some mitochondria, ribosomes and endoplasmic reticulum. In 1-2 days, they develop into erythrocytes and enter circulation.
Regulation of erythropoeisis:-
The level of oxygen in the tissue is the principle regulatory factor of erythropoietic activity of the bone marrow. The kidney cells, during hypoxia, releases erythrogenin (erythropoietin releasing factor) from the glomeruli, which in turn acts on erythropoietinogen, an µ2 globulin of plasma and converts it into free erythropoietin (hemopoietin). Kidney produces 90% of erythropoeitin and liver produces about 10%.
Erythropoietin as a hormone stimulates hemopoietic stem cells of bone marrow to produce the committed stem cells-proerythroblast, thus initiates erythropoiesis. It stimulates,
The proliferation of rubriblast by mitosis in the developing rubricytes.
Accelerates maturation of the rubricytic cells.
Induces the release of reticulocytes into the circulation.
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